The invention concerns a method for the production of phase-stabilized particulate ammonium nitrate (AN) by incorporating a diamine complex of the metals copper, nickel or zinc into the crystal lattice of the AN, wherein the AN is melted and reacted with inorganic copper, nickel and/or zinc compounds.
Ammonium nitrate is used as oxidant in propellants and explosive substances, in gas generators, in rockets and recently also in air bags. While AN disintegrates with slow heating at temperatures above its melting point of 169.5xc2x0 C. into water and dinitrogen monoxide, disintegration at higher temperatures or under rapid heating (exothermal) can occur as a detonation thereby releasing oxygen, nitrogen and nitric oxides.
Pure AN has five different crystal modifications depending on the temperature, i.e. cubical in the temperature interval between 125xc2x0 C. and its melting point of 169.5xc2x0 C. (modification 1), tetragonal in the temperature interval between 84xc2x0 C. and 125xc2x0 C. with a density of approximately 1.67 g/cm3 (modification II), orthorhombic in the temperature interval between 32xc2x0 C. and 84xc2x0 C. with a density of approximately 1.66 g/cm3 (modification III), also orthorhombic between xe2x88x9218xc2x0 C. and 32xc2x0 C. (modification IV), but with a density of approximately 1.73 g/cm3 which represents a density change of approximately 4% compared with the density of modification III, and orthorhombic pseudotetragonal (modification V) at temperatures below xe2x88x9218xc2x0 C. Modifications II and V are thereby very similar with respect to their density and their lattice structure and provide almost identical diagrams in X-ray diffraction measurements.
In particular, the density difference between the modifications III and IV during heating of AN to temperatures above 32xc2x0 C. produces tension and the formation of cracks in the structure of the formed charges containing AN as oxidant.
Different types of additives have been proposed to phase-stabilize AN. It is e.g. known to stabilize modification III in the temperature interval between xe2x88x9220xc2x0 C. and 100xc2x0 C. through the addition of alkaline nitrates, such as potassium nitrate. The AN particles mixed with nitrates, however, tend to bake together and thereby exhibit poor flow and are difficult to homogenize in mixtures (U.S. Pat. No. 3,018,164). Moreover, modification III is not sufficiently stabilized through the addition of potassium nitrate, in particular when AN, mixed with potassium nitrate, cools down to temperatures below approximately xe2x88x9230xc2x0 C.
Phase stabilization, which was successful in practice, of the similar modifications II and V and modification IV in a temperature interval of approximately xe2x88x92100xc2x0 C. to 100xc2x0 C. is achieved by adding metal ammoniates (metal ammine complexes), preferably via ammine complexes of the metals copper, nickel and zinc (DE 17 67 757 A1, DE 21 25 755, EP 0 405 272 B1).
Production of metal ammine complexes or integration in the crystal lattice of AN is effected e.g. through melting a mixture of AN and metal oxide (DE 21 25 755 C3). AN is thereby mixed with up to 10% of the metal oxide, is melted and the molten mass is transferred into a solid state. This method has the disadvantage that the ammine complex is insufficiently integrated into the AN lattice since the reaction between metal oxide and AN is very slow and the ammine complex disintegrates at an increased temperature, i.e. the melting temperature of ammonium nitrate. The formation frequency and simultaneous disintegration thus compete with one another and residues of the non-reacted metal oxides remain in the phase-stabilized product which have a negative influence on the combustion of ammonium nitrate.
EP 0 405 272 B1 discloses a method in which the diamine complex is produced through reaction of the metal oxide with AN in a solid-state reaction at 110xc2x0 C. to 170xc2x0 C. This method is also demanding and expensive since the diffusion rate of the solid-state reaction is low and there are residues of non-reacted metal oxides in the AN. Both lead to an inhomogeneous product. DE 36 42 850 C1 describes a method wherein melted AN is reacted with copper and/or nickel oxide thereby forming the respective diamine complexes (equations 1 and 2).
2NH4NO3+CuOxe2x86x92[Cu(NH3)2]2++2NO3xe2x88x92+H2Oxe2x80x83xe2x80x83(1)
2NH4NO3+NiOxe2x86x92[Ni(NH3)2]2++2NO3xe2x88x92+H2Oxe2x80x83xe2x80x83(2)
The molten mass must be prepared in small spatially separated charges for safety reasons to be subsequently processed in a spraying device into spherical phase-stabilized AN particles.
According to U.S. Pat. No. 5,071,630 A1, zinc oxide is used for forming the diamine complex instead of copper and/or nickel oxide. Zinc oxide is added to AN in the molten phase, the molten mass is dried with inert gas and charged with ammonia to replace the NH3, discharged during drying. Subsequently, the molten mass is optionally dissolved in pure ammonium nitrate and the obtained product is cooled and granulated.
The metal oxides (CuO, NiO and ZnO) are thereby also disadvantageous since they react only slowly with AN and the formed diamine complex starts to disintegrate at temperatures above the melting temperature of AN. Small amounts of non-reacted nickel oxide or copper oxide therefore always remain in the product which have a negative influence on the burning behavior of AN. The metal oxides deposit in the reaction container and the production plant must be frequently cleaned to prevent transport of the deposited oxides into the subsequent charge. This increases the cost of the method. Moreover, contaminations, in particular, toxic nickel oxide cannot be excluded.
It is the underlying purpose of the invention to improve the known method by reducing the reaction time required for forming the diamine complex, preventing soiling of the product through non-reacted metal oxides, improving the quality of the product and reducing production costs.
In accordance with the invention, this object is achieved with the above-mentioned method in that carbonates and/or hydroxide carbonates and/or hydroxides and/or hydroxide nitrates are added as inorganic compounds of the metals copper, nickel and/or zinc.
The inventive compounds, having considerably less toxic substances than those of the corresponding oxides, achieve a reaction of almost 100% with considerably less reaction time such that there are no residues of the compounds in the product and in the reaction container. A highly pure phase-stabilized AN with exact specification is obtained.
While the diamine complex of nickel (II) stabilizes the modification IV of AN, the diamine complexes of copper (II) or zinc (II) stabilize the modifications II or V. In any case, the modification of AN stabilized in this fashion is stable within a large temperature range of approximately xe2x88x92100xc2x0 C. to 70xc2x0 C.
The large reaction yield achieved through adding copper, nickel or zinc carbonates, hydroxide carbonates, hydroxides, or hydroxide nitrates in accordance with the invention is based on the special reaction behavior of the carbonate or hydroxide anions and on the reduced stability compared to the corresponding metal oxides due to reduced lattice energy. The carbonate or hydroxide anions react spontaneously with the ammonium ion of AN thereby releasing ammonia (equations 3 and 4).
OHxe2x88x92+NH4+xe2x86x92NH3+H2Oxe2x80x83xe2x80x83(3)
CO32xe2x88x92+2NH4+xe2x86x922NH3+CO2xe2x80x83xe2x80x83(4)
The produced ammonia reacts again with the respective cations of the metals Cu, Ni or Zn thereby forming the diamine complex.
The gross reactions of the hydroxides, carbonates and hydroxide carbonates with AN are exemplarily summarized in equations 5 through 7 for the copper cation.
2NH4NO3+Cu(OH)2xe2x86x92[Cu(NH3)2]2++2NO3xe2x88x92+2H2Oxe2x80x83xe2x80x83(5)
2NH4NO3+CuCO3xe2x86x92[Cu(NH3)2]2++2NO3xe2x88x92+CO2+H2Oxe2x80x83xe2x80x83(6)
4NH4NO3+Cu(OH)2.CuCO3xe2x86x922[Cu(NH3)2]2++4NO3xe2x88x92+CO2+3H2Oxe2x80x83xe2x80x83(7)
Since copper carbonate (equation 6) is difficult to obtain in its pure form, the use of double salts such as CuCO3.K2CO3 is possible. Instead of pure hydroxides, hydroxide nitrates such as 3Cu(OH)2.Cu(NO3)2, Ni(OH)2.Ni(NO3)2.6H2O or 4Zn(OH)2.Zn(NO3).2H2O can also be added. AN is thereby not soiled with a foreign anion.
A further preferred embodiment provides that when the reaction of AN with the inventive additives for phase stabilization shall take place in the liquid phase, additives are added for reducing the melting temperature of AN to reduce the danger of explosion and the heating up time. The process may then be carried out in one single, optionally also large-volume reaction container such as e.g. heatable kneading devices, screw mixers or revolving cylindrical furnaces and must not be carried out, for safety reasons, in several small charges in parallel or one behind the other. The charge change can be considerably accelerated since the plant components do not have to be cleaned from soiling sediments. The inventive method can even be carried out continuously when corresponding technology is used for the system.
While in a preferred variant, additives can be added which reduce the melting point of AN according to Raoults"" laws, it is possible to provide in particular such additives which form, in small concentrations in the AN molten mass, an eutectic mixture, to utilize a melting point in the eutectic range which is lower than that of the pure substances.
Preferably, water and/or metal nitrates are added. Water has the advantage that it volatilizes e.g. during spraying in a cooled gas sink flow. The advantage of metal nitrates is based on the fact that the ammonia nitrate is only soiled by the cation of the added metal nitrate if a different cation is selected than the cations copper, nickel or zinc added for phase-stabilization of AN in the form of carbonates, hydroxide carbonates and/or hydroxides. For this reason, in particular copper, nickel or zinc nitrate is used. If the quality of the phase-stabilized AN is not impaired by the presence of further cations, other nitrates, e.g. alkaline or alkaline earth nitrates such as NaNO3, Ca(NO3)2 or Mg(NO3)2 can also be added. While the hydroxides, carbonates or hydroxide carbonates of the metals copper, nickel or zinc are added in an amount corresponding to the desired portion of the diamine complex in the product, the amount of the substances lowering the melting point may, optionally, be small. The proportion of the diamine complex with phase-stabilized AN for use in gas generators of rockets is 2 to 5 mass %, for use as propellant for air bags up to 50 mass %.